This invention relates to a method of manufacturing a semiconductor device and, in particular, a method of manufacturing a semiconductor device having a stacked-gate-electrode structure.
An EPROM (Erasable Programmable Read Only Memory) includes a two-layered gate electrode structure: a floating gate and control gate. Conventionally, the floating gate is formed by etching a polycrystalline silicon layer which is formed on a first gate insulating film. In this connection it is to be noted that an opening is formed by an etching step in the polycrystalline silicon layer. At the same time, there occur cases where protruding ends are formed on the polycrystalline silicon layer. Thereafter, a conductive layer is so formed that it overlies the etched polycrystalline silicon layer with a second gate insulating film therebetween. In this way, a control gate is formed.
According to the conventional method, there occur cases where no such second gate insulating film of an adequate thickness is formed at the ends of the opening and thus the critical field strength of the second gate insulating film is lowered. Furthermore, if the end of the etched polycrystalline silicon layer protrudes, an electric field is concentrated there, thus lowering the critical field strength of the second gate insulating film. If the conductive layer is formed at the location of the opening, there is disadvantage that a breakage occurs at the stepped portion of the opening. This phenomenon prominently occurs if the conductive layer, in particular, is formed of, for example, a metal layer having a high melting point, a layer of a silicide of the metal having high melting point or a polycide layer (i.e., a two-layered structure comprised of a polycrystalline silicon layer and a layer of a silicide of the metal having a high melting point).
U.S. Pat. No. 4,412,310 to George J. Korsh et al. discloses the process of forming an oxide layer between a floating gate and an overlying control gate, in which the oxide layer is thicker at the edges of the floating gate than in the central portion thereof. According to this invention, however, the above-mentioned problem is solved by another method.